From fiberglass to plywood, composite materials have become quite common in man-made materials for construction, fabrication and the like. A polymer composite includes at least one polymer matrix or material in combination with at least one particulate filler material. The polymer matrix material may be any of a number of polymers including themoplastics such as polyamide (Nylon), poly-urethane, polyolefins, vinyl polymers, and the like, thermosets, and elastomers. Some of the most common nanoparticle fillers are two-dimensional nanoclays, one-dimensional carbon nanotubes, and zero-dimensional metal oxide nanoparticles such as Zinc Oxide (ZnO), Titanium Dioxide (Ti02), and Zirconia (ZrO). Today, composite materials can be found in various products such as automobiles, building materials, food packaging and textiles. Composites offer the potential of materials having properties that are not often available in naturally occurring raw materials. Whether an ultra-lightweight material is needed with the strength to reinforce structural components or a transparent flame retardant material, (e.g., U.S. Pat. No. 6,518,324 to Kresta et al. for a Polymer Foam Containing Nanoclay), such characteristics are often the result of a composite material.
Traditional polymer composites have several potential limitations. First, the amount of filler required to impart the desired property enhancement is often significant—possibly in the range of about 30-60% by weight (weight percent). This creates several problems, including the potential for a composite that is heavier than the base polymeric material. It is sometimes the case that the expense of a large quantity of filler adds significant material cost to the final product, with the possibility of surpassing an economic threshold. In addition, blending, or pre-compounding, of the polymer with high levels of fillers may be necessary prior to forming the final product, adding an additional step and additional cost. Perhaps more importantly, the addition of fillers may also result in deleterious effects on the ductility or strength of the composite and thereby compromising the mechanical characteristics of the composite. One such phenomena is embrittlement, which occurs when any agglomeration or defect in the uniform dispersion of the filler particles creates an inhomogeneity and causes a weak point in the composite.
Second, the interaction between the filler and the matrix is critical to the properties of any composite. A filler particle can have several effects on the surrounding polymer matrix. If there is strong adhesion between the particle and the polymer, the polymer strength and modulus are typically increased, often at the expense of elongation. If the adhesion between the filler and the polymer is weak, the polymer strength may actually decrease, resulting in elastomeric-like properties, as described, for example, by Benjamin J. Ash et al., in “Investigation into the Thermal and Mechanical Behavior of PMMA/Alumina Nanocomposites,” Materials Research Society Symposium Proceedings, Vol. 661, p. KK2 10.1-6 (2001), which is hereby incorporated by reference in its entirety.
Third, effective filler materials often increase the potential environmental impacts of composite products, for example, further increasing lifecycles of plastics, including those disposed of in landfills and, depending on the nature of the filler, potentially increasing harmful effluent leakage into local resources (e.g., the chromate fillers used in anti-corrosion coatings). In summary, the material tradeoffs are often prohibitive or less than ideal for utilization of known composites.
One particular class of composite has demonstrated great promise in overcoming the limitations and tradeoffs noted above—polymer nanocomposites. Nanocomposites generally include one or several types of nano-scale particles dispersed within a polymer matrix. The benefits of nanoparticles are derived from the very large surface area interactions of the nanoparticles with the polymer matrix. The nature of this interaction allows for beneficial property improvements, sometimes using fillers at very low loading levels, often as low as about 1 to 10 weight percent. The possibility of using lower loading levels reduces the concerns relative to increased waste and increased costs. The lower loading levels also increases the potential for homogeneous dispersion of the filler within the composite matrix. One exciting advance observed with nanocomposite research has been the ability to combine the properties of the polymeric matrix with those of the nanoparticle filler with little to no trade-offs—so that the nanocomposite may be both strong and ductile. The implications of this discovery extend the possibility of creating multifunctional composites, which may exhibit improved strength, ductility, corrosion resistance, flame retardation, and optical transparency.
Nanocomposites are not exempt from traditional challenges of other well-known composites because the advancement of nanocomposites requires both matrix/filler compatibility and the effective dispersion of filler within the matrix. If either of these requirements is not achieved, the properties of the nanocomposite will likely suffer, even perhaps becoming less effective than the corresponding macro-composite or the polymeric matrix material. Therefore, much of the work surrounding nanocomposites is directed to attaining homogenous mixtures and finding ways to assure the filler is functionalized to interact with the matrix.
A significant portion of the nanocomposite materials on the market today are based upon nanoclay fillers. In general nanoclay fillers consist of laminar clays, some of which are naturally occurring (e.g., kaolin and smectite), and synthetic clays, (e.g., fluorohectorite and fluoromica). Each of the nanoclays is a layered silicate, held together by an intercalation layer—often water. The nanocomposite filler consists of “exfoliated” two-dimensional sheets of clay. In some embodiments, the individual layers are separated from one another and dispersed throughout a polymer matrix. The exfoliation, or separation, process is quite complex and often incomplete, thus frequently leaving larger pieces of clay that create weak points in the polymer matrix. Exfoliation generally involves first swelling the clay by introducing small interacting molecules or polymers into the intercalation space existing between the clay layers, to increase the distance between layers, and finally introducing a shear force or energy to complete the separation of the layers.
As silicates are naturally hydrophilic and many industrially important polymers are hydrophobic, the clay may also be modified or functionalized before mixing the two together while seeking to disperse the filler in the polymer matrix. Otherwise the filler and matrix will phase separate rather than form a homogeneous composite. Moreover, organic surface modifiers, used to increase the binding between filler and matrix often adversely affect the properties of the composite.
Exfoliation is generally accomplished using one of three processes. One process is melt intercalation of the polymer into an organically modified silicate. Although this process works well with more polar polymers such as polysiloxanes and polyethers, less polar polymers such as polypropylene, requires a modifier, such as maleic anhydride, often in the form of a maleic anhydride-polypropylene copolymer, to compatibilize the nanocomposite. In addition, modification of the layered clay material may also be required. A second process is the formation of a layered silicate in an aqueous polymer solution. The development of this process will likely be limited to polymers that are soluble or dispersible in water. A third process that is receiving increasing attention involves a silicate that is intercalated by an initiator or catalyst, and upon introduction of a monomer an intercalated or exfoliated polymer nanocomposite is formed. See, for example, Bergman, J. S., Chen, H., Giannelis, E., Thomas, M., Coates, G., Chem. Comm. (1999) 2179-2180, which is hereby incorporated by reference in its entirety.
Exfoliation can be quite challenging and expensive, due to the addition of the extra processing step(s). Often, even the best processes do not fully exfoliate the clay samples, but rather only the outermost or top several layers. In such situations, un-exfoliated clay samples may become incorporated into the nanocomposite, causing inhomogeneity and weak points throughout the polymer composite matrix. The exfoliation challenge leads to difficulty in obtaining a homogeneous dispersion, thereby producing a polymer composite with particles that tend to re-agglomerate and resist separation.
The present disclosure addresses these weaknesses in current nanoclay composites while providing additional functionality, or multifunctionality, to these composites that is not currently available with two-dimensional nanoclay composites. Disclosed embodiments include those directed to polymeric composites including nanoclays, particularly those utilizing mineral nanotubes, and a method for preparing such composites. The advantages are at least two-fold and include ease of processing (no need for exfoliation) as well as a geometry that provides acceptable binding of the tube to the polymer matrix. Furthermore, the use of the nanotubes provides additional functionality via the inner open space or cavity of the tube, particularly the ability to incorporate active chemical agents within the tubes, or to coat the tube surfaces.
Disclosed in embodiments herein is a polymeric nanoparticle composite, comprising: a polymer matrix; and a filler consisting essentially of mineral nanotubes.
Also disclosed in embodiments herein is a method for making a polymer composite, including: producing an air milled treated halloysite having a nanotubular structure; and combining a polymer material with said surface treated halloysite to form the polymer composite.
Further disclosed in embodiments herein is a polymer composition including about 1 to about 10 weight-percent of a nanotubular clay filler dispersed therein, based on the total weight of polymer, said nanoclay filler including nanotubes having an outer cylindrical diameter of less than about 500 nm and a length of less than about 40,000 nm (40 um).
Further disclosed in embodiments herein is a method for producing a polymer composite part, including: obtaining a tubular clay filler material; surface modifying the tubular clay filler material; air milling the surface treated material; drying the material; combining the dried material with a polymer to form a composite mixture; and forming the composite mixture into the part.
The various embodiments described herein are not intended to limit the invention to those embodiments described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.